1. Field of the Invention
The embodiments of the invention generally relate to phase locked loops, and, more particularly, to a phase locked loop (PLL) that includes a voltage controlled oscillator band selection circuit.
2. Description of the Related Art
A PLL is a close-loop system which requires each component unit with high linearity to keep the system stable; however, a voltage controlled oscillator (VCO) is a non linear unit: the output frequency is not a linear function of the input control voltage because of the following. First, in the ring oscillator based VCO, the source-drain current is not a linear function of the gate voltage. Secondly, in a LC tank based VCO, the capacitance of a varactor is not a linear function of the voltage across it and the resonance frequency of the LC tank is a square root function of the capacitance. Thirdly, the VCO gain can vary by more than 2.5 times over the control voltage range. One way to mitigate this is to have a multiband VCO and choose the VCO band for a desired frequency so that the VCO gain stays relatively constant.
For the wireless communication application, the PLL has a VCO with multiple frequency bands (up to 16 bands or more). Different VCO bands might be implemented by using a set of the programmable fixed capacitors parallel to the varactor. During the startup procedure one of the VCO bands is chosen and the control voltage is monitored during the closed loop operation of the PLL. If the control voltage is within a specified range this VCO band is selected. Otherwise, next VCO band is selected and the PLL loop is closed. The control voltage is monitored and if it is within the specified voltage range, this VCO band is selected. This procedure is repeated till the correct VCO band is chosen. One disadvantage of such a system is that it usually takes more than 2 ms to complete one band selection, and the overall band selection procedure could be very time consuming.